The present invention generally relates to gaming apparatus and, more particularly, to electronic reel-type slot machines having a plurality of reels rotatable about a common axis. In a typical reel-type slot machine, a payoff is made to a player when a winning set of symbols is displayed on the pay line(s) of the machine. To start play, a button is pushed or a handle is pulled to initiate rotation of the reels.
In one type of design, the angular positions of the reels, after they have been stopped, is detected and the appropriate payoff amount, if any, is calculated and paid to the player. Another approach in modern machines uses a random number generator to select the symbols to be displayed on the pay line(s). The payoff is then determined based on a pay table which contains payoff amounts for the various winning symbol combinations. Payoff amounts provided by either approach are limited because there is a fixed limit on the probability of obtaining the maximum payoff, which is the reciprocal of the number of reel stop positions per reel raised to the power of the number of reels.
Accordingly, it is desirable for manufacturers of reel-type slot machines to provide new ways to increase reel-type slot machine payoff values while maintaining adequate game revenue for the operator. As the payoff amounts increase, player interest in the game is fostered which leads to maximized game revenue.
One method of increasing payoff values in a prior art electronic slot machine design is to employ a "virtual reel". According to this method, a plurality of numbers are assigned to most of the physical reel stop positions and at least one number is assigned to every physical reel stop position. In this way, the chances of winning the larger payoffs can be decreased by assigning these stop combinations to fewer numbers.
The present invention provides an alternative method for increasing payoff levels in electronic reel-type slot machines. The odds of obtaining a particular winning symbol set can be "dialed in" by assigning each possible reel stop combination to a unique terminal node (position) in a random number fractional branching tree. The tree comprises a main tier, a plurality of lower tiers and a plurality of terminal nodes. Each of the tiers has a number of entries which lead either to a lower tier or to a terminal node.
During game play, one of the entries on the main tier is randomly selected by the game microprocessor. If the randomly selected entry leads to a lower tier, then one of the entries on that tier is randomly selected. This selection process continues for each successive tier until a terminal node is selected. One reel stop combination is assigned to each terminal node. The combination assigned to the selected terminal node is then displayed on the pay line(s) of the slot machine. A payoff is made to the player if the combination displayed corresponds to a winning symbol combination in a posted pay table.